A. Field of the Invention
The present invention relates to a wet friction plate within a wet power transmitting and interrupting mechanism or clutch mechanism, where the wet friction plate is provided with a cushioning member.
B. Description of the Background Art
In a wet power transmitting and interrupting mechanism such as a wet clutch and a wet brake, a wet friction facing used in a wet friction plate is made of a paper or organic material or the like.
For instance, a wet multiple plate clutch arranged in a transmission is used for changing direction and speed of torque transmission. The multiple plate clutch includes a plurality of drive plates and a plurality of driven plates which are arranged alternatively to each other in the axial direction. Each driven plate is provided at its opposite surfaces with wet friction facings, e.g., of an organic material fixed thereto. One problem with such devices is that the surfaces engaged by the drive and driven plates are expensive to manufacture in that they must be very finely machined to be flat or planar.
A lockup clutch of a torque converter is a clutch device for transmitting a torque by mechanically coupling a front cover and a turbine together, and is provided for improving a fuel consumption of a vehicle. The lockup clutch is formed of, e.g., a piston which can be coupled to the front cover and a damper mechanism for coupling the piston and a member at the turbine side together. A wet friction facing made of organic material is fixed to a side of the piston opposed to the front cover.
The damper mechanism of the lockup clutch absorbs a torque vibration caused by vibration in combustion in an engine. However, it is impossible to operate the lockup clutch in a low speed range of the vehicle, because a torque vibration at a level, which cannot be sufficiently absorbed by the damper mechanism, is generated in this low speed range. In recent years, slip control has been utilized for further improving a fuel consumption by operating the lockup clutch in a lower speed range. The slip control is performed by pressing the piston against the front cover with a small coupling force, and thereby steadily allowing controlled slippage between the piston and the front cover. When the controlled slippage is utilized, torque is transmitted through divided paths, i.e., a mechanical transmission path (the slipping surfaces) and a hydraulic transmission path. When the controlled slippage is large, torque is mechanically transmitted at a small rate, and is hydraulically transmitted at a large rate. When the controlled slippage is small, a power is mechanically transmitted at a large rate, and is hydraulically transmitted at a small rate. The controlled slippage speed is controlled by a hydraulic pressure control device which controls a difference between hydraulic pressures at opposite sides of the piston in the torque converter.
In a lockup clutch and a wet multiple plate clutch in the prior art, it is difficult to keep uniform contact in circumferential and radial directions between the wet friction facing and the friction surface. In other words, partial contact is liable to occur. This often results in the following and other problems.
(1) A shudder is liable to occur when the clutch is engaged.
(2) A wet friction facing is liable to wear to a higher extent.
(3) In the lockup clutch, leak of a lockup hydraulic pressure is liable to occur.
For reducing a partial contact, the opposed friction surface must have a high flatness, which requires a high processing accuracy and therefore increases a cost.
The slip control of the lockup clutch in the torque converter suffers from the following problem. In the prior art, the slip control is performed, for example, in a vehicle speed range from 30 to 48 km/h with a fourth gear position. Although it is preferable to perform the slip control in a lower vehicle speed range for improving a fuel consumption, this results in the following difficulties.
In a low speed range, it is necessary to change a ratio of the power transmitted through a hydraulic path to a large extent, so that it is necessary to increase the slip rotation speed to a certain extent. In order to increase the slip rotation speed to a certain extent, a difference between hydraulic pressures at opposite sides of the piston member must be extremely reduced. When the pressure difference is finely controlled in this state, the piston may suddenly move toward the front cover due to change in hydraulic pressure difference. When this occurs, the torque variation is mechanically transmitted to the output member. For the above reason, it is difficult to perform the slip control at a low speed range, e.g., under 30 km/h.